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Topic: Boomerang wah pot reversal (Read 12422 times)

After studying an old Dunlop shell compared with an original Boomerang it struck me that the pot rotation could be reversed by bolting in the bracket from a Stagg wah pot. These are sometimes sold quite inexpensively, complete with the pot and gear. The casing needs to be cut away and the bypass switch moved. A hairpin spring also needs to be bent up from spring wire (called piano wire here in England). The whole action is a lot smoother than the original Dunlop and the rotation increases by a few degrees. It more closely resembles the Boomerang mechanism.

The pot is the pedal's original - pre-HotPotz. Don't know the taper but the value is 100k. I found that adding resistors in an attempt to trim it to 25k didn't work as well as the pot on its own. I'd like to get a 25k pot to see what difference it makes. The common practice of adding supplementary resistors is flawed, as it doesn't really address what the wiper 'sees' in either direction. I tried a 33k resistor off each leg to wiper, making roughly 25k total at either end of travel, and 19k from the wiper to each end at the centre of travel. I wasn't convinced it sounded any better, just different.

Perhaps a standard 25k audio-taper pot may make a (short-lived) substitute, though many Colorsound wahs appear to use a standard pot and these last a while.

It would, but then the pedal would work in reverse - heel down would be the treble position.

The feel, tonal sweep and pedal travel are much improved over a standard setup. Also, the plastic P clip on a Dunlop used to keep the rack in contact with the pinion limits the heel down travel. With this setup the rear rubber stop can be pared down to get more pot rotation - the rack will keep engaged down to the very last tooth.

There must be some reason why the original Boomerang uses this setup, while pretty much every other manufacturer installs the pot as per Dunlop. Perhaps the reversal optimizes the pot's curve in relation to the frequency response of the pedal circuit.

mick, are you the guy that modded that mrbillyhill's pedal? if so, nice job, that thing sounds excellent- very close to the freq response and richness of the nice originals.

either way, here we go:

modding a 100k pot down to 25k via limiting resistors does not do the correct job in a boomer circ. i found this on my own thru hundreds of experiments/measurements, but also had this independently verified by the man who designed the BG2, richard mintz ( he commented on this without any prompting from me as i was asking about other aspects of the boomer).

his exact words about the taper of the original BG2 centralab pots were " they were reverse audio taper, but slightly modified". keep in mind, in the late 60s or early 70s, if gibson, inc asked for a spec; ial pot taper, you as a mfgr would provide said taper. he later told me that the volume pedal or wah function would be compromised if a correct taper pot was not used in that circ, as the biasing and the whole circ was tuned exactly for that pot spec. he also said that the original centralabs would vary from 22k up to 28k. every vintage unit i have tested hits very near 30k with original pot.

i think the stagg might be similar to the washsburn/lyons wah unit i have here. however, it had a similar gear mech to a crybaby, so there may have been different versions. the pot in that thing looked like cheap chinese crap, i would not have trusted it for a gig, even if the the gig was only one song long. (every chinese 25mm pot i have seen made for wah is short life, including propot, the recent dunlop chinese made wahs, etc etc.)

to your Q about a standard 25k audio doing the job, if you do this, go back to the crybaby rotation, this will be more correct to the original sweep direction.

just saw the pics of your re-work. very nice. unfortunately, that pot looks like one of he ones that will last about 4 to 7 songs before going scratchy. hop i am wrong tho.

paul, no, just switching the direction of the pot by changing wiring does not do it. mick was correct in actually physically making this pot go the other way. in my experiments, the hump is totally in the wrong place if simply reversing the wiring, hence the dozens of hours making 'cam gears' to make hotpotz sound correct in these ( 33k limiting Rs included). we got close, but when zac came over and watched my foot movement and listened to the frequencies, " close" was still not as good as a real 72 boomerang we were AB'ing to. i have a vid showing this. will post it.

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my life is a tribute to the the great men and women who held this country together when the world was in trouble. my debt cannot be repaid, but i will do my best.

as a reference, hotpotz1 and hotpotz2 have a taper that looks very much like audio, but with a more pronounced dead spot on the high end ( i believe this was spec'd by thomas long ago to make the switching better, easier to adjust on multiple units with fewer warranty claims), dunlop has never changed the 70s hotpotz spec as far as my measurements of many units can find. the hotpotz2 is around 10k lower on average, but the ramp is near identical to any 72 -78 unit from thomas.

so in answer to this thread, yes, a boomerang, with opposite rotation would naturally be rev audio or rev log. mick did a good thing here.

« Last Edit: February 20, 2012, 01:38:49 PM by joegagan »

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my life is a tribute to the the great men and women who held this country together when the world was in trouble. my debt cannot be repaid, but i will do my best.

here is the zac zagar boomerang clone. the pot is going in standard CB rotation. pot is a hotpotz1, but with a special offset gear to mimic sweep of original+ it has limiting resistors, i believe 33k.

the frequency of a boomerang is there, but the feel of the pedal is not the same. plus i believe that there is an actual parametric shelving action happening on either side of the two halves of the pot that is different from a real 25k pot. my theory has to do with the time quotient, ie; charge/discharge rate of the cap to ground off the inductor relative to the resistance of both sides of the pot at any given point in time as the player moves the pedal.

the vid does not show the comparo to a real boomerang, unfortunately, the test i referred to wa not on video.

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my life is a tribute to the the great men and women who held this country together when the world was in trouble. my debt cannot be repaid, but i will do my best.

if you listen just to the frequency curve, you can see in this well made BG clone, the same basic sweep. i have had similar experiences when brian temblay sent me soundsamples of his noomerang ( ROG gauss markov tweaked BG typ wa).

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my life is a tribute to the the great men and women who held this country together when the world was in trouble. my debt cannot be repaid, but i will do my best.

paul, no, just switching the direction of the pot by changing wiring does not do it. mick was correct in actually physically making this pot go the other way. in my experiments, the hump is totally in the wrong place if simply reversing the wiring.

OK, now I get it. Getting my foot out of my mouth now... Hmm... maybe I need to get out my Boomerang clone and tweak it.

Yes - I modded MrBillyHill's pedal - he borrowed mine and liked it enough to buy a pretty decent Dunlop donor pedal for me to convert.

The pot in my own is stamped on the cover 'Pro Pot 0349'. It's a little scratchy now, but has done quite a few years of gigging/rehearsals prior to my ownership and eases of after a few minute's playing. I suppose some Deoxit may help. Or a new pot.

I've only ever bought the Stagg pots for the brackets, as the track runout on the pot is extremely noisy so the full travel can't be used in either direction. They seem to be broken from new. I got caught the last time I bought some, though - I just got the pots and no brackets, so need to engineer something myself.

MrBillyHill's donor pedal had a rather nice inductor, which I momentarily coveted as I unsoldered it. A two-piece affair which can be unscrewed and re-wound. Who ever has inductor envy? Is this how it gets you? Would love to try an El-Rad or similar to further improve the pedal. I tried the green Fasel from my old 70s Jen and couldn't really detect any significant improvement. Need to find a cheap source of ferrite cores and press my old coil winder back into service and do some experiments.

As it stands, both pedals have got that classic 70s funk 'chop' sound when the pedal is quickly rocked back from the toe-down position with each chord, something I've found lacking in Vox and Dunlop pedals. Proper 'Shaft' stuff.

Just to be clear to anyone looking in at these posts - I can't claim any real innovation here - I've just applied Joe's experiments, advice and research to my own situation and based the build on Paul Marossy's excellent pedal, which is inspirational. Interestingly, I've been playing around with wah circuits for a couple of years and now have a graveyard of failed experiments/vero/PCBs which didn't do the trick. These things can take quite a while to tweak.

I've only got twenty good summers left, so need to work a little faster from now on......

It is a standard rotation pot, though I didn't measure the value or notice what type it was when I built it - will measure it next time I see him. There are no compensating resistors fitted and it works very well as it is. It was intended as a temporary arrangement until I could get some of the Stagg brackets. Surprisingly, when I did an A-B test with my own pedal the sweep was so close it didn't warrant the extra work.

On the face of it, the Stagg pots are pretty well made; they have a moulded thermosetting plastic body with a moulded-in threaded bush. The contacts are epoxied over the rivets and the shaft is accurately machined, with a well-fitting pinion. The cover fits tightly to prevent ingress of dust. However, when you rotate them they sound like an old wire wound rheostat, or like it was filled with sand. I haven't taken one apart to inspect the wiper arrangement or track. I had a Shin-Ei phaser that had an identical looking pot fitted from new and it looks like the Stagg is a copy of that component.

Shame, really - these could have been a really good, cheap replacement.

I've just pulled the cover off a Stagg pot. The first thing I noticed was the impressive track width. The second was that the wiper only contacts about 1/4 of this width.

The mechanical rotation is far greater than the electrical rotation - the wiper leaves the track by quite some distance at either end of travel. Now I know why these won't operate to the full extent of mechanical rotation.

The gritty noise/feeling is the wiper scratching the track. The design may have been inspired by a cat using a litter tray. Looks like it needs some Deoxit right from new - D100?

Bit of a grainy pic attached showing the wiper having left the track and how the wiper fingers don't make the best use of the track width;

makes me happy to know that some of my posting resulted in another gearhead giving the boomerang a go.

I built three or four of them and they never knocked me out. What schematic were you using?

You should try reversing the pot like Mick did.

You know, it just occurred to me that I could use one of several 100K reverse log pots that someone sent me about five years ago in my Boomerang clone. That way I wouldn't have to modify the Crybaby shell at all and it might have a similar feel and sound to the real Boomerang. Maybe I'll do that tonight, or Wed night. I'll let y'all know what I find...

paul, i have had much better results using an unchoked 100k in the dearmond 1800 instead of the bg. there are minor differences but they seem to make the circ more forgiving of pot value. the differences are:

2.2 uf caps off pot instead of 1 uf

820k to ground from base on Q2 for biasing ( not on BG)

270k from Q1 collecor instead of 1.5M

33k to ground at Q2 emitter instead of 8.2k.schematic:

i built a modded version of this, but will have to look up my notes. gus smalley and i were brainstorming the day i did the mods. you can hear the mod version here. it is smooth and not trebly. the subbing of a 100k pot makes the top end less intense. the rich mid emphasis reminds me of a boomerang, but with less treble at toe down.

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my life is a tribute to the the great men and women who held this country together when the world was in trouble. my debt cannot be repaid, but i will do my best.

mick, thanks for that report. there are some guys in Uk selling a wah pot made in china that looks very much like that. i have also bought some chinese 500k for volume pedals that look much like that, but i have never opened them up.

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my life is a tribute to the the great men and women who held this country together when the world was in trouble. my debt cannot be repaid, but i will do my best.

paul, i have had much better results using an unchoked 100k in the dearmond 1800 instead of the bg. there are minor differences but they seem to make the circ more forgiving of pot value.

Oh yeah, the Boomerang uses a 25K pot. Geez, too much on my mind lately with all this stuff I have to do since my father passed away.

Anyway, just for kicks I'll put a 100K reverse log pot in my Boomerang clone in a Crybaby shell to see what it might sound like. I'm not looking to make an exact clone of the real Boomerang, but if it gets me closer in sound and feel then I'll be happier with it. I like my BG-2s, but they are so freakin' huge that I just can't put them on my pedalboard. A Crybaby sized pedal just works much better for me.

I built three or four of them and they never knocked me out. What schematic were you using?

Paul Marossy's BG2 schematic and layout, with the components removed to mod it to BG1. Transistor selection is important - doesn't seem to work very well with anything other than low-gain types. I also increased the 6uf cap as per Joe Gagan's findings - mine comes out at 7.9uf (1 + 2.2 + 4.7). Hence my little sub-board where a single cap should live.